Electrochemical hardness modification of non-allotropic metal surfaces

ABSTRACT

An electrochemical method of modifying the surface hardness of a non-allotropic metal member 10, comprising: (a) forming the member to near net-shape with at least one surface 12 to be hardened; (b) subjecting the surface 12 to rapid melting and resolidification by incidence of an electrical discharge between an electrode 16 and the surface 12 closely spaced thereto, the spacing containing an electrolyte with plasma forming capability, the surface 12 being hardened by crystallographic change of the globules resulting from substitutional alloying; and (c) cropping the surface grains 29 of the surface to increase load bearing capacity while retaining liquid retention capacity.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to technology for modifying the surface hardnessof metal parts that have a near net-shape form, and more particularly toelectrochemical techniques for achieving such hardness modification.

2. Discussion of the Prior Art

Selective surfaces of Ferrous based articles have been hardened bymelting the surface with high energy, such as by electron bombardment,laser light, or plasma stream, and allowing the body of the Ferrousmetal to chill the melted surface to produce a phase hardened surface.Metal surfaces have been hardened by thermal chemical treatment whereinmolecules from an electrode or from a surrounding gas medium isimpregnated into the metal surface. Surfaces have also been hardened byadhesion of superimposed films of harder material.

High energy beams are disadvantageous because they are difficult toregulate, expensive to operate and often require safety measures toprotect the user. Thermal chemical treatments require a delicate andsophisticated energy producing apparatus in a tightly enclosed chamberwhich makes the system difficult to use and is expensive. Adherentlayers of harder material often complicate and distort the nearnet-shape of the article so that it is more difficult to achieve anexact final shape of the article without increasing the cost ofmanufacturing.

Applicant is unaware of hardening of non-allotropic metals, such asaluminum, by electrochemical treatment wherein an electrical dischargeacross an insulative dielectric fluid causes globules of thenon-allotropic metal surface to melt and upon removal of the electricaldischarge, the globules are allowed to resolidify with alloying elementsin the dielectric or metal surface forcing substitutional alloying and aharder surface. Applicant is aware of an electrochemical process, oftenreferred to electrical discharge machining, that has been used toprogressively remove surface metal from articles but with no attentionto controlling hardness of the resulting work piece surface.

SUMMARY OF THE INVENTION

The invention, in a first aspect, is an electrochemical method ofmodifying the surface hardness of a non-allotropic metal member,comprising: (a) forming the member to near net-shape with at least onesurface to be hardened; (b) subjecting the surface to rapid melting andresolidification by incidence of an electrical discharge between anelectrode and the surface closely spaced thereto, the spacing containingan electrolyte with plasma forming capability, the surface beinghardened by crystallographic change of the globules resulting fromsubstitutional alloying or solid solution strengthening; and (c)cropping the surface grains of the surface to increase load bearingcapacity while retaining liquid retention capacity.

The invention, in another aspect, is a unitary aluminum based swashplatemember useful in a compressor, comprising: (a) a plate drivinglyrotatable about an axis through its center but canted to the plane ofthe plate; (b) integral shoulders on opposite sides of the plate, eachpresenting a thrust surface for receiving a plurality of rolling bearingloads, the thrust surfaces being centered about such axis and being in aplane normal to such axis; and (c) each thrust surface having (i) ahardness enhanced thermochemically by electric discharge to a depth of10-400 microns, and (ii) a surface roughness of 1.5 MmRa or less, thethrust surfaces being effective to substantially reduce the cost ofswashplate fabrication and reduce load bearing failures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a compressor swashplate formed to nearnet-shape as the first step of the inventive process;

FIG. 2 is a highly enlarged schematic cross-section of the thrustbearing surface of the swashplate as a result of the first step;

FIG. 3 is a schematic illustration of an apparatus for carrying out thesecond step of the inventive process;

FIG. 4 is a highly enlarged schematic cross-section on the same scale asin FIG. 2, showing the condition of the thrust surface after the secondstep of the process;

FIG. 5 is a representation of a scanning electron micrograph of a planview of the thrust bearing surface after the second step of the process;

FIG. 6 is a representation of a scanning electron cross-sectionmicrograph of the same surface as in FIG. 5;

FIG. 7 is a highly enlarged cross-section, on the same scale as in FIG.2, showing the condition of the thrust bearing surface after the thirdstep of the process;

FIG. 8 is a bargragph showing the variation of swashplate worn areavolums as a function of resulting hardness for differing heatthermochemically treated specimens under two differing loadingconditions; and

DETAILED DESCRIPTION AND BEST MODE

The method of this invention comprises essentially three steps, thefirst of which is to form a metal member 10 of non-allotropic metal 11to near net-shape with surfaces 12, 13 that will be subject to highrolling or rubbing stresses and therefore need to be hardened. Formingmay be carried out by casting, machining from wrought bar stock, or byforging. As shown in FIG. 1, the member is a compressor swashplateformed from 390 aluminum alloy by forging. Near net-shape is used hereinto mean that critical surfaces, such as 12 and 13, are substantiallymade to finish shape within 3.5 Mm. The starting roughness of suchsurfaces is usually about 2.0 MmRa, when forged, or about 1.0 MmRa whenrough machined to near net-shape. As shown in FIG. 2, the surfaces willhave peaks 14 and valleys 15 of substantial difference.

Non-allotropic metals include aluminum, magnesium and titanium. Suchmetals must contain alloying ingredients that are capable of promotingsolution hardening by crystallographic change (the alloying ingredientstraining the molecular matrix of the metal). For example, in aluminum,silicon, copper, magnesium, iron and manganese serve this purpose andmay be present in cast aluminum alloys of 319, 390, 356, 357,380 and inwrought aluminum alloys of the 2000, 3000, 6000 and 7000 series. Suchaluminum alloying ingredient should be present in an amount of at least0.15% by weight and contain as much as 15% in some alloys. Formagnesium, the ingredient can be Al, Zn, Mn, Si, Cu, Ni, or Fe; fortitanium, the ingredient can be Al, V, Fe or Sn.

The starting surface hardness of such near net-shape member is aboutR_(b) 40-55 when cast of aluminum or when rough machined from wroughtaluminum. For a magnesium and titanium member, such hardness is aboutR_(b) 35-45 and R_(b) 65-75 respectively.

The second step of the process is to subject the surfaces 12 and 13 torapid melting and resolidification by incidence of an electricaldischarge between an electrode 16 and the surface 12 and 13 which isclosely spaced thereto. The spacing 17 should contain an electrolyte 18with plasma forming capabilities so that the surface can be hardened bycrystallographic change of globules resulting from rapid melting andwhich globules undergo substitutional alloying or solid solutionstrengthening. One or more electrodes 16 are shaped complementary to thesurfaces 12 and 13 and are arranged to be positioned within about 40micrometers of such surfaces. The electrodes may be carried ormanipulated by a robotic arm 19 to facilitate the rapid cycling of theelectric discharge step. A suitable power supply 20 feeds electricalcurrent to the electrodes 16 according to a programmed scheme. Themedium of the electrolyte 18 fills the gap 17 existing between theelectrodes and the surfaces to the modified. The electrolyte isintroduced into the gap when the electrode is immersed in the liquid oftank 21. Thus, the necessary components for an electrical discharge tooccur across the sparking gap 17, for purposes of this method, requiresapplication of a DC voltage to a cathodic electrode, connecting themetal member 10 to act as an anode in the dielectric fluid; thedielectric fluid 18 can be deionized water with a typical conductivityof about 15 microsiemens. The deionized water may contain cations ofhydrogen, sodium, calcium, magnesium, aluminum, iron and anions, such ashydroxides, chlorides, bicarbonates, carbonates, sulfates, nitrates andphosphates. Common contaminants in deionized water include sodium,silica, carbon dioxide and bicarbonate. It is usual to have metalspresent in deionized water such as iron, copper.

At the initiation of electric discharge, there is at first no electriccurrent flowing between the anodic member surface 12 and the cothodicelectrode surface 22. Current will pulse initially due to the insulationof the water dielectric in the gap 17. Within a few microseconds, anelectric field will cause the micron impurities particles to besuspended and form a bridge across the gap 17 which then results in thebreakdown of the dielectric. The voltage will fall to a lower level andthe current will increase to a constant value as adjusted by theoperator. Due to the emission of negative particles, a plasma channelwill grow during the pulse "on" time. A vapor bubble will then formaround the plasma channel and the surrounding dense water dielectricwill restrict plasma growth, concentrating the input energy to a verysmall volume. The plasma temperature will reach very high levels, suchas 40,000 k and the plasma pressure can rise to as much as a 3 k bar.There will be a melting-reshaping of metal globules at the surfaces 12or 13 as a result of the reduced heat input after drop in the currentperiod. As the current flow halts, the bubble implodes therebydistorting the molten globules without freezing them. The dielectricfluid solidifies this molten material by its temperature differentialbefore such material can be carried away. The cycle is repeated during asubsequent "on" time of the current cycle.

Because of bombardment by fast moving electrons at the start of thepulse, the surface to be hardened as globules which will melt rapidlyfirst but then begin to resolidify after a few microseconds.

To insure the conditions for hardness enhancement, the voltage promotingthe electrical discharge should be in the range of 5-10 volts max., theamperage should be in the range of 3-20 amps, and the discharge pulseshould be "on" for periods of 200-1000 microseconds. The duration overwhich the hardening treatment is carried out is usually about 0.5-2minutes. The voltage/amp period is kept considerably lower than thatused for roughening or for electrical discharge machining. The depth ofhardness can be varied with a slight increase in voltage and pulse.

As the result of the second step, the surface 12 treated by theelectrical discharge will have a smoother, but undulating profile asshown in FIG. 4. New peaks 23 and new valleys 24 are reduced byrelocation of the melting and rapid resolidification. The affectedsurface, to a depth 25, will be enhanced in hardness to about R_(b)65-80. Roughness can be tailored by manipulating voltage, amperagepulsation, or the electrical discharge process. Evidence of moreuniformity in the surface character of the affected swashplate is shownin the scanning electron micrographs of FIGS. 5 and 6. FIG. 5 shows thesurface uncoated as resulting from electric discharge. FIG. 6 is asectional scanning electron micrograph of a coated surface previouslysubjected to electric discharge showing the depth of the affected layerto be 200-900 microns deep. A high degree of mechanical interlock takesplace between the coating 26 and the cropped electrically discharged andchemically modified surface 27.

The third step of the process is to crop along a plane 28 the surfacegrains 29 of the surface 12 to increase its load bearing capacity, asshown in FIG. 7. This may be carried out by honing, using a diamond flatwheel that crops the tops of the peaks of the surface grains. Thesurface roughness will be reduced to 1.5 Ra or less without affectingthe hardness previously imparted as a result of the electrical dischargetreatment.

The wear characteristic of a 357 aluminum alloy member can be determinedby subjecting the member to a block on ring wear test. The resultingdata is shown in FIG. 8 wherein Group A bars represent wear volumes forspecimens that were subjected to a dry wear test at 36,000 psi, andGroup B bars represent specimens subjected to a lubricated wear test at36,000 psi. Group C bars represent specimens subjected to a dry weartest at 10,000 psi, and Group D bars represent specimens subjected to alubricated wear test at 10,000 psi. The wear data for lubricated Group Bspecimens decrease significantly as the hardness is increased. Groups Cand D are for specimens that were both run dry and lubricated under a10,000 psi load; under this lighter loading, the increase in hardness ofthe specimen again shows a definite trend towards reduction of wearwhether it be dry or lubricated.

The resulting new product, such as a compressor swashplate, possessesseveral new advantages. First, the swashplate product may eliminatefailure due to galling and sliding wear. Secondly, the cost of makingthe compressor swashplate is substantially reduced as a result ofsurface hardening from the electrical discharge process when compared toconventional hard coating applications used to prevent wear. Inoperative use, such as shown in the partial compressor assembly in FIG.9, the swashplate 10 is rotatably drivingly mounted about an axis 30through its center that is canted to the plane 31 of the plate. Shoes32,33 on opposite sides of the plate have a plurality of seats 34 eachcradling a bearing 35 which present a rolling or sliding load on thethrust surfaces 12 or 13 centered about axis 30. The thrust surfaceshave a hardness enhanced thermochemically by electric discharge to adepth of about 100 Mm and each have a surface roughness of 1.5 MmRa orless. The thrust surfaces are effective to substantially reduce the costof swashplate fabrication and reduce load bearing failures.

While particular embodiments of the invention have been illustrated anddescribed, it will be obvious to those skilled in the art that variouschanges and modifications may be made without departing from theinvention, and it is intended to cover in the appended claims all suchmodifications and equivalents as fall within the true spirit and scopeof this invention.

We claim:
 1. An electrochemical method of modifying the surface hardnessof a non-allotropic metal member comprising:(a) forming said member tonear net-shape with at least one surface to be hardened; (b) subjectingsaid surface to rapid melting and resolidification by incidence of aplasma of an electrical discharge between an electrode and said surfacewhich is closely spaced thereto, the spacing containing a dielectricfluid with plasma forming capabilities, the surface being hardened bycrystallographic change of the globules resulting from substitutionalalloying or solid solution strengthening; and (c) cropping the surfacegrains of said surface to increase load bearing capacity while retainingliquid retention capacity.
 2. The method as in claim 1, in which thehardness of said treated surface is increased by at least 25 HK.
 3. Themethod as in claim 1, in which the electrical discharge is carried outat low voltage and amperage.
 4. The method as in claim 1, in which thedepth of surface hardening is increased by increasing the voltage andthe pulse period of said electrical discharge.
 5. The method as in claim1, in which the discharge of step (b) is carried out with a voltage inthe range of 5-20 volts and the discharge being pulsed for periods of200-1000 microseconds.
 6. The method as in claim 1, in which theroughness of the cropped hardened surface is 1.5 MmRa or less.
 7. Themethod as in claim 1, in which said metal member is selected from thegroup consisting of titanium, magnesium and aluminum.
 8. The method asin claim 7, in which said metal is aluminum selected from the groupconsisting of cast aluminum alloys 319, 390, 356, 357, 380 and wroughtaluminum alloys of the 2000, 3000, 6000 and 7000 series.
 9. The methodas in claim 1, in which said member is constituted of a metal withsubstitutional alloying ingredient present therein.
 10. Anelectrochemical method of modifying the surface hardness of anon-allotropic metal member comprising:(a) forming said member to nearnet-shape with at least one surface to be hardened; (b) subjecting saidsurface to rapid melting and resolidification by incidence of anelectrical discharge between an electrode and said surface which isclosely spaced thereto, the spacing containing an electrolyte withplasma forming capabilities, the surface being hardened bycrystallographic change of the globules resulting from substitutionalalloying or solid solution strengthening; and (c) cropping the surfacegrains of said surface by diamond flat honing to increase load bearingcapacity while retaining liquid retention capacity.